Dread wasting time? That’s a mistake (5)
I believe the answer is:
error
'that's a mistake' is the definition.
(synonyms)
'dread wasting time?' is the wordplay.
'dread' becomes 'terror' (synonyms).
'wasting' is a deletion indicator.
'time?' becomes 't' (abbreviation).
'terror' with 't' removed is 'ERROR'.
(Other definitions for error that I've seen before include "Trial and ___" , "Miscalculation" , "A mistake or inaccuracy" , "wrongdoing" , "Boob" .)